Although high-luminance projectors are provided that take mercury lamps as a light source, mercury lamps contain mercury, a hazardous material, and taking into consideration the impact upon the environment, there is consequently an urgent need for the development of mercury-free products. Against this background, the development of LEDs (Light Emitting Diodes) and LDs (Laser Diodes) has been advanced, and low-luminance LED projectors and laser projectors that take these devices as light sources are already being introduced into the marketplace.
However, because high-luminance green LEDs or compact, high-output green LDs are not yet being mass-produced, a high-luminance LED projector or laser projector has not yet been introduced to the market.
In response, high-luminance projectors that use a phosphor as a light source have been proposed. In Patent Documents 1 and 2, projector light source devices that employ phosphors are described.
The light source device described in Patent Document 1 has a solid-state light source unit, a condensing unit, a diffuser, a dichroic mirror, a condenser lens, a wave plate, a reflecting plate, and a fluorescent light-emitting plate.
The solid-state light source unit is equipped with a plurality of blue semiconductor lasers that are arranged in array form. A collimator lens is provided for each blue semiconductor laser, and the blue light supplied from each blue semiconductor laser is converted to parallel luminous flux by each corresponding collimator lens. The solid-state light source unit is configured to emit both S-polarized blue light and P-polarized blue light.
The blue light emitted from the solid-state light source unit is irradiated into a dichroic mirror by way of a condensing unit and diffuser. The condensing unit is made up of a first lens that has positive power (also referred to as positive refractive power) and a second lens that has negative power (also referred to as negative refractive power), the first lens being arranged on the solid-state light source unit side. The first and second lenses reduce the luminous flux diameter of the blue light from the solid-state light source unit. The diffuser diffuses the blue light from the condensing unit.
The dichroic mirror has the property of transmitting light that is irradiated as P-polarized light and reflecting light irradiated as S-polarized light in the blue wavelength band, and further, transmitting light of the green and red wavelength bands. Of the blue diffused light from the diffuser, the S-polarized light is reflected by the dichroic mirror and irradiated into the condenser lens. On the other hand, the P-polarized light is transmitted by the dichroic mirror without alteration and irradiated into the wave plate.
The blue reflected light (S-polarized light) from the dichroic mirror is condensed upon the fluorescent light-emitting plate by the condenser lens. The fluorescent light-emitting plate is a rotatable disk-shaped component and includes a phosphor layer. The phosphor layer includes a red phosphor region that produces red fluorescent light and a green phosphor region that produces green fluorescent light, the blue light being successively irradiated upon the red phosphor region and green phosphor region by the rotation of the fluorescent light-emitting plate. The red fluorescent light from the red phosphor region and the green fluorescent light from the green phosphor region each irradiate the dichroic mirror by way of the condenser lens. The red fluorescent light and green fluorescent light pass through the dichroic mirror.
The transmitted blue light from the dichroic mirror (P-polarized light) passes through the wave plate and is irradiated upon the reflecting plate. The reflecting plate reflects the irradiated blue light in the direction of the wave plate. The reflected blue light from the reflecting plate again passes through the wave plate and is irradiated upon the dichroic mirror. The transmitted blue light (P-polarized light), by twice passing through the wave plate, is converted to S-polarized light. The blue light from the wave plate (S-polarized light) is reflected by the dichroic mirror. The reflected blue light (S-polarized light) from the dichroic mirror is emitted from the light source device on the same optical path as the red fluorescent light and green fluorescent light that were transmitted through the dichroic mirror.
The light source device described in Patent Document 2 includes a plurality of blue LDs (laser diodes), a plurality of collimator lenses, a condenser lens, a concave lens, a dichroic mirror, a condenser lens group, and a phosphor wheel.
A collimator lens is provided for each blue LD, and the blue light supplied from each blue LD is converted to parallel luminous flux by each corresponding collimator lens. The blue light emitted by way of each collimator lens from each blue LD is irradiated by way of the condenser lens, the concave lens, the dichroic mirror, and the condenser lens group on a phosphor layer of the phosphor wheel on which a green phosphor is formed.
Each blue LD and each collimator lens are arranged such that the irradiated regions on the phosphor layer of the blue light from each blue LD mutually overlap.
In the projectors provided with the light source devices that are described in Patent Document 1 and Patent Document 2, the fluorescent light that is produced by the phosphor is irradiated by way of an illumination optical system on a display element such as a digital micromirror device (DMD) or a liquid crystal display element. The image that is formed by the display element is projected on an outside screen by a projection optical system.
When the irradiation size of the excitation light is reduced, the light density per unit area increases, whereby the peak light intensity of the excitation light increases. Conversely, an increase in the irradiation size of the excitation light lowers the light density per unit area, and the peak light intensity of the excitation light therefore decreases. The peak light intensity of excitation light is thus determined by the irradiation size of the excitation light.
The emitted light intensity of a phosphor typically depends on the light intensity of the excitation light. However, when the light intensity of excitation light reaches a particular value, the phenomenon of saturation or decrease occurs according to which the emitted light intensity of the phosphor does not increase despite any further increase of the light intensity of the excitation light, and the wavelength conversion efficiency from excitation light to fluorescent light therefore drops. As a result, the irradiation size of the excitation light must be set such that this type of phenomenon does not occur (first limitation).
In addition, the emitted light size of the phosphor region (phosphor layer) depends on the irradiation size (spot size) of the excitation light. Increasing the irradiation size of the excitation light increases the light emission size, thereby complicating the efficient capture of the fluorescent light by the illumination optical system or projection optical system. As a result, the irradiation size of the excitation light must be set such that the fluorescent light can be reliably captured by the illumination optical system and projection optical system (second limitation).
Normally, the irradiation size of excitation light is set to a size suitable for satisfying the above-described first and second limitations.